Plasma ion source having apertured extractor cathode



Oct. 25, 1966 J. D. H. WOOD PLASMA ION SOURCE HAVING APERTURED EXTRACTOR CATHODE 6 Sheets-Sheet 1 Filed Feb. 19. 1963 41 blx BQI 7 11 6 1 8 E 2 WM 3 T A C R O T flu A R T X E D E R U T R E P A G N I V A H E G R U 0 S N o I A M S A L F Oct. 25, 1966 J. D. H. WOOD 6 Sheets-Sheet Filed Feb. 19, 1963 JoV/Sidd O a &

Oct. 25, 1966 J. D. L. H. WOOD 3,231,617

PLASMA ION SOURCE HAVING APERTURED EXTRACTOR CATHODE Filed Feb. 19, 1963 6 Sheets-Sheet 4 50- 200/ 504 55 pam W Targe/ carren/ 1/. ,O/fJSV/ 1, MM LA'VAZ M7774 5M4 H 3 T A C R O T aw A R T X E D E R U T R E A G N I v A H E C R U o s N O I A M S A L P J. D. L. H. WOOD Oct. 25, 1966 6 Sheets-Sheet 6 Filed Feb. 19, 1963 FIG] 2E QM United States Patent 3,281,617 PLASMA ION SGURCE HAVING APERTURED EXTRACTOR CATHODE James David London Hedley Wood, Waysbroolr, Letchworth, England, assignor to United Kingdom Atomic Energy Authority, London, England Filed Feb. 19, 1963, Ser. No. 259,497 Claims priority, application Great Britain, Feb. 20, 1962, 6,503/ 62 6 Claims. (Cl. 31361) This invention relates to ion sources suitable for use in apparatus for carrying out a nuclear reaction by bombarding a target material with high energy ions in an ion beam, the ions being produced as a plasma in an ion source and accelerated across an acceleration gap which contains gas at substantially the same pressure as gas in the ion source.

Apparatus of the above type does not require pumping means to keep the gas pressure low in the acceleration gap and as a result the ion source and the acceleration gap can be contained within a common sealed envelope.

The apparatus has its prime use as a neutron generator, in which case the favoured nuclear reactions are the DT and DD reactions and a gas pressure controller is provided within the sealed envelope to compensate for gas used in the nuclear reaction or lost by absorption on solid surfaces.

In apparatus of this type it is essential to have an ion source which can operate at very low gas pressures down to microns, and the most suitable ion sources depend on the ionization of the gas by collision with electrons. Since the mean free path of the electrons is large at the low gas pressures which must be used, it is essential to provide some means for containing the electrons within the ion source for a time sufiiciently great to produce ionizing collisions and generate a plasma which consists of the gas in a highly ionised state.

Ion sources which have been used up to now consist essentially of two cathodes which face each other from opposite sides of the ion source, a cylindrical anofde positioned between the cathodes with its ends facing the cathodes and a magnet or electromagnet for producing a magnetic field which penetrates the plasma and has the same direction as the axis of the cylindrical anode. The two cathodes generate a potential trough in which electrons can swing to and fro inside the cylindrical anode, and the magnetic field forces the electrons into helical paths and increases the lengths of the paths they travel before capture by the anode.

The plasma is an electrically conducting medium, consisting as it does essentially of positive ions and electrons, and electrostatic field lines therefore terminate at its boundary and do not extend into its interior. The magnetic field can penetrate the plasma and therefore applies a constraint to the whole of the plasma thereby exerting its optimum elfect and it has been universally regarded as essential for the constraint to be applied as uniformly as possible within the plasma.

The magnetic field has up to now been the only known means for preventing charged particles escaping and diffusing to the walls of the ion source and to constrain the directions of movement of electrons. This magnetic field can be best provided either by a permanent magnet inside the sealed envelope or by an electro-Inagnetic coil positioned outside the sealed envelope.

There are disadvantages with both the above types of magnet arrangennent. Strong permanent magnets are normally made of sintered metal and they contain significant quantities of gas in their voids. Such gas tends to be released under the low-pressure conditions obtaining in the generator, and can introduce undesirable impurities which can seriously worsen the generator performance.

The external arrangement adds to the bulk of the tube and introduces complications into the design of the ion source since special construction of the tube is needed to avoid screening preventing the entry of the magnetic field into the interior of the ion source and the plasma.

This invention provides an ion source arrangement which does not require the use of a magnetic field and therefore avoids the above disadvantages.

The invention consists in an ion source for generating a plasma, which comprises a cathode, an extractor cathode spaced from the said cathode and having an aperture therein, an anode positioned between the cathode and the extractor cathode and separated by a gap therefrom, and a screen negative electrode positioned about the said cathode and external to the plasma, and extending in a direction towards the extractor cathode to apply an electric field to the gap between the cathode and the anode.

The elfect of the screen negative electrode is most unexpected in vieW of the known properties of plasmas. At the low gas pressures normally used the plasma extends to within microns of the cathode and therefore the electrons emitted by the cathode are swallowed almost immediately by the plasma and should be screened from the field of the screen negative electrode. It is apparent that the anode itself must screen the plasma from any external electric field and it would be expected that the behaviour of the plasma would therefore be determined only by the field exerted by the anode and cathode and that the system would behave simply as an ion source having no magnetic field. As is well known from Pennings work one would expect the gas pressure required to produce ions in any useful quantity to be about 1000 times the pressure which could be used in the presence of the magnetic field.

In contrast to this it has been found that the application of the electric field as stated above has a powerful influence on the whole ion source and that the magnetic field can be completely dispensed with.

The ion source can be either the cold cathode type or the hot cathode type. With the col-d cathode type each cathode would be provided with a screen negative electrode.

An example of the invention is illustrated in the accompanying drawings in which:

FIG. 1 is a sectional elevation of a neutron generator,

FIG. 2 illustrates plasma impedance at 20 micron deuterium pressure,

FIG. 3 illustrates variations in plasma voltage and screen volts,

FIG. 4 illustrates the relationship of target current to gas pressure,

FIG. 5 illustrates the relationship between target current and target volts,

FIG. 6 shows the neutron output as a function of target voltage, and

FIG. 7 shows the neutron yield as a function of screen negative electrode volts.

In FIGURE 1 an oxide hot cathode at earth potential I mounted on supporting leads 2 is surrounded by a cylindrical screen negative electrode 3 mounted on a connecting lead 4. A cylindrical anode 5 is carried on a support lead 6. All support leads pass through a pinch 7 of a glass envelope 8. An extractor cathode 9 at earth potential is mounted between the anode 5 and a target 10. Leads 11 are connected to the target 10 which consists of tritiated titanium supported on a molybdenum base. A dispenser 12 (described and claimed in my specification No. 850,950) for deuterium is contained in a glass extension 13 and a Pirani gauge 14 in another glass extension 15, the interiors of which communicate directly with the space inside the envelope 8.

A transparent plastic container 16 protects the glass envelope 8 and a corona shield 17 covers one end of the container 16.

The generator described is more suitable for use as a continuous generator than as a pulsed generator.

The DC. operation of the tube was investigated.

Operating levels investigated were in the range:

Pressure-l to 40 microns Plasma current--0.4 to 2.0 milli-amps Plasma volts-+9 to +35 with respect to the cathode Screen volts--2 to200 with respect to the cathode Target voltsl0 to 90 kv. with respect to the cathode.

Ion source variations are shown in FIGURE 2, screen volts varying from -100 to 200 volts and target volts from 50 to 90 kv. It will be seen that up to 1 milli-amp plasma current, the impedance across the plasma, increases at a steady rate. Above 1 ma. the impedance is fairly constant up to 1.7 ma., above 1.7 ma. the impedance falls. Removal of the target volts appears to make no appreciable change in the plasma impedance.

FIGURE 3 shows the variation of plasma voltage and screen current with screen volts, for 1 milli-amp plasma current at pressures of 10 to 40 microns. It should be noted that no target volts were applied during the period readings were taken.

FIGURE 4 shows variation of target current at 10 to 40 microns pressure. The screen was maintained at 200 volts.

FIGURE 5 shows variation of target current with voltage. It will be observed that the chart divides itself into two groups. The three high level runs were obtained when the plasmacurrent was set between 1.5 to 2.0 ma. The lower three curves were taken with the level of plasma current between 1.1 and 1.3 ma. There is a small difference in the rate of slope between the two sections, which cannot readily be explained.

FIGURE 6 shows neutron yield as a function of target voltage. Consideration should be given to the fact that the ion beam decreases in diameter, with increasing screen negative electrode volts. It is clearly an advantage to contain the ion source into a diameter equal to that of the target, because of the increased efficiency of the tube. It is possible, however, to increase the screen potential until ultimately the ions are concentrated into a pencil beam, that operates over a small area of the target surface, thereby causing damage.

FIGURE 7 shows the neutron yield as a function of screen voltage. The target voltage was 50 kv., the plasma current was set at 1 ma., and the gas pressure was 20 microns. It can be seen that the neutron yield rises as the screen negative electrode voltage was varied from 25 to -100 v.

A run at 90 kv. target voltage and 200 v. on the screen negative electrode produced a neutron yield of neutrons/ sec.

It is instructive to compare these yields with the yields obtained with the neutron generator described in Review of Scientific Instruments, vol. 31, No. 3, March 1960. The similarities between the design of the neutron generator described therein and the design of the neutron generator discussed in this specification are apparent. The difference between them resides essentially in the fact that the neutron generator in the above reference uses a magnetic field coil to apply constraint to the plasma. The above reference describes the performance of the generator in pulsed operation only. In DC. operation the neutron yield with kv. target voltage, 1 ma. plasma current and 200 gauss magnetic field was found to be 10 neutrons per second. This is of the same order of magnitude as obtained with the generator embodying the present invention.

It will be realised that the conditions given above, particularly the screen negative electrode voltages, are appropriate primarily for the geometry of the equipment described and that slightly different conditions would apply to different geometries. These can be found by routine experiment.

I claim:

1. A plasma ion source comprising a cathode, an apertured extractor cathode spaced from said cathode, a cylindrical anode located between said cathode and said extractor cathode and separated by gaps therefrom, and a screen negative electrode extending over the gap between said cathode and said anode to apply a constraining field.

2. A plasma ion source comprising a cathode, an apertured extractor cathode spaced from said cathode, a cylindrical anode located between said cathode and said extractor cathode and separated by gaps therefrom, and a cylindrical screen negative electrode extending over the gap between said cathode and said anode, said cathode being located within one end of the cylindrical screen negative electrode and said anode extending within the other end.

3. A plasma ion source as claimed in claim 2 wherein said cathode is a thermionic cathode.

4. A neutron generator comprising an envelope enclosing a target containing a hydrogen isotope and adapted to produce neutrons when bombarded with accelerated hydrogen isotope ions and a plasma ion source for providing a beam of said hydrogen isotope ions, said source comprising a cathode, an apertured extractor cathode located between said cathode and said target, a cylindrical anode located between said cathode and said extractor cathode and separated by gaps therefrom, and a screen negative electrode extending over the gap between said cathode and said anode to apply a constraining field.

5. A neutron generator as claimed in claim 4 wherein said screen negative electrode is a cylinder having said cathode located within one end thereof and said anode extending within the other end thereof.

6. A neutron generator as claimed in claim 5 wherein said cathode is a thermionic cathode.

References Cited by the Examiner UNITED STATES PATENTS 2,831,134 4/1958 Reifenschweiler 3l323l X 3,090,882 5/1963 Benway 3l382 FOREIGN PATENTS 1,268,230 6/1961 France.

OTHER REFERENCES Hawkins et .al.: Compact Pulsed Generator of Fast Neutrons, Review of Scientific Instruments, vol. 31, Am. Inst. of Physics, N.Y., 1960, pp. 241-248.

JAMES A. LAWRENCE, Primary Examiner.

GEORGE N. WESTBY, Examiner.

R. L. JUDD, Assistant Examiner. 

1. A PLASMA ION SOURCE COMPRISING A CATHODE, AN APERTURED EXTRACTOR CATHODE SPACED FROM SAID CATHODE, A CYLINDRICAL ANODE LOCATED BETWEEN SAID CATHODE AND SAID EXTRACTOR CATHODE AND SEPARATED BY GAPS THEREFROM, AND A SCREEN NEGATIVE ELECTRODE EXTENDING OVER THE GAP BETWEEN SAID CATHODE AND SAID ANODE TO APPLY A CONSTRAINING FIELD. 